Change the type of in_hsfraglen to unsigned

In the `mbedtls_ssl_context` structure, change the type of `in_hsfraglen`
from `size_t` to `unsigned`. This is in preparation for merging
`in_hsfraglen` into `badmac_seen_or_in_hsfraglen`, which has the type
`unsigned` and cannot change since we do not want to change the ABI.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/include/mbedtls/ssl.h b/include/mbedtls/ssl.h
index 0cad449..161eb7e 100644
--- a/include/mbedtls/ssl.h
+++ b/include/mbedtls/ssl.h
@@ -1826,7 +1826,7 @@
 
     size_t MBEDTLS_PRIVATE(in_hslen);            /*!< current handshake message length,
                                                     including the handshake header   */
-    size_t MBEDTLS_PRIVATE(in_hsfraglen);        /*!< accumulated length of hs fragments
+    unsigned MBEDTLS_PRIVATE(in_hsfraglen);      /*!< accumulated length of hs fragments
                                                     (up to in_hslen) */
     int MBEDTLS_PRIVATE(nb_zero);                /*!< # of 0-length encrypted messages */